Re: Help: PCLEX, PCYACC and reserved words

Garry Whitworth wrote:
> 2nd Question: The language I am dealing with also allows the
> programmer to declare a variable using one of its reserved word


> [Yuck. Sometimes you can get away with an ugly grammar that says
>
> symbol | thiskeyword | thatkeyword | theotherkeyword
>
> all over the place, or maybe you can identify the contexts well enough in
> the parser to tell the lexer when it's expecting a keyword and when it
> isn't. -John]


This is indeed an extremely disgusting problem. A possible way to deal
with this is to write a piece of code that emulates yacc to see if the
keyword is acceptable at that point. The way to use this code is to
have it check every keyword for acceptablity (it should not cause a
parse error immediately) and if not, convert it to a regular
identifier. I could post the code for the bison engine, but I don't
know how well (if at all) it would apply to other yacc clones. The
trick is to copy the state stack, and perform reduces until the
keyword must be shifted or causes an error. If that happens, the
keyword can not be accepted at this point and has to be converted to
an identifier. Unfortunately, this code does require reading of the
parse engine, which has not been written with easy reading in mind.




M. Thiehatten


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